Electrical resistance heating element for a heating device for heating a flowing gaseous medium

ABSTRACT

An electrical resistance heating element for a heating device, with at least one flow canal through which the gaseous medium is able to flow from a canal inlet side to a canal outlet side of the resistance heating element, and with at least one heating resistor that extends essentially in the direction of the flow canal, with the medium flowing past the heating resistor and being heated in the process. The heating resistor is rod-shaped and produced from an electrically conductive ceramic material. The electrical resistance heating element is intended for installation in a heating tube into which air, for example, is blown at one end. The heated flow of air exits from the other end of the heating tube.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority under 35 USC §119 to European Patent Application No. 08 010 426.8, filed on Jun. 9, 2008, the entire disclosure of which is incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The invention concerns an electrical resistance heating element for a heating device for heating a gaseous medium, with at least one heating resistor that extends essentially in the longitudinal direction of the electrical resistance heating element, with the medium flowing past the heating resistor and being heated in the process. The resistance heating element has at least one flow canal that extends along the heating resistor and through which the gaseous medium is able to flow from a canal inlet side to a canal outlet side of the resistance heating element, with the heating resistor consisting of an electrically conductive ceramic material for conducting the current. The invention also relates to a heating device for a flowing gaseous medium that is equipped with an electrical resistance heating element of this type.

DESCRIPTION OF THE RELATED ART

In this context, heating devices specifically means heating apparatus, modules, or systems where the electrical resistance heating element is located in a heating tube. Air or gas, for example, are blown into one end of the tube, and the heated flow of air or gas exits from the other end. The air flow may be generated by a blower provided on the heating device, or may be supplied to the heating device by an external air flow generator, or in case of a gas flow, it may be supplied by a preferably pressurized gas reservoir.

Such heating devices are commonly known and are used extensively in industry and trade. In these devices, in order to generate a hot flow of air or gas, a cold flow of the gaseous medium is guided towards the resistance heating element and is then guided through and/or along the heating element exposed to the flow, with the cold medium being heated increasingly on its way from the inlet side to the outlet side of the heating resistor due to its contact with the hot heating resistor. The flowing gaseous medium passes the heating element in one or several flow canals that enclose at least one heating resistor.

From DE 198 39 044 A1, such a heating device is known in the form of a hot air device where the resistance heating element is located in a heating tube. Commonly, various nozzles can be attached to the side of the heating tube that is associated with the air outlet side of the heating element. The heating element comprises a carrier made of a ceramic material of high thermal strength that is held, via a central pin, on a connection head located on a side facing the blower. This connection head also serves for the electrical connection of the heating resistor that is formed by heating wires extending in a spiral configuration in a central air canal. A thin ceramic cover plate, also held by the central pin, is located at the air outlet side of the heating element. The connection head as well as the cover plate have passages for the flow of air, enabling it to pass unimpeded through the air canal with the heating resistor.

If the resistance heating elements used there are operated without a sufficient flow of air for a certain amount of time, the heating wire will usually burn through, thereby rendering the heating element unusable. An insufficient flow of air may be caused, for example, by a failure of the blower or a restriction of the air inlet or air outlet cross-section of the hot air device. For this reason, special protective electrical measures need to be taken to protect the heating wire from undesirable overheating. For this purpose, commercial hot air devices are commonly equipped with a thermal sensor or switch that reduces or interrupts the supply of power to the heating resistor when there is a danger of overheating.

Regarding prior art, reference is made to the publications DE 100 12 675 A1, U.S. Pat. No. 6,442,341 B1, and EP 0 899 985.

DE 100 12 675 A1 discloses an electrical flow-though resistance heating element made of PCT ceramics that, with a small cross-section and low weight rates of flow, is especially well suited for heating liquids flowing through it. In the flow direction, the heating element has an oblong sectional body that, in the longitudinal direction, has an essentially constant cross-sectional area and layers of electrodes by which the heating current is conducted through the walls of the sectional body essentially perpendicular to the flow direction of the flowing liquid. Throughout, the walls have an essentially even thickness, corresponding to the current path, and heating ribs equipped with electrode layers that intrude into the flow canal.

U.S. Pat. No. 6,442,341 B1 also discloses a flow-though heater for liquids. The flow-through heater disclosed there has a resistance heating element embedded in a metallic housing. In the housing, a flow canal for the liquid runs parallel, at a distance, to the one heating element. The resistance heating element itself has no integrated flow canals and is in direct contact with the housing with its entire surface. The heat from the heating element is transferred indirectly to the flowing liquid along a section of the housing in which the tube-shaped flow canal is located, with the housing being made of a material of good thermal conductivity.

EP 0 899 985 A1 relates to a flow-though heater for heating a liquid in a closed circulation system of a motor vehicle. The flow-through heater comprises at least one heating body with at least one canal through which the liquid to be heated flows, and with at least one PCT heating element for heating the heating body positioned in close vicinity. In its structure and function, it is essentially similar to the flow-through heater disclosed in U.S. Pat. No. 6,442,341 B1. It differs from this by having several flow canals and several heating elements.

SUMMARY OF THE INVENTION

The invention addresses the problem of proposing an improved electrical resistance heating element for a heating device for a flowing gaseous medium, wherein the danger of overheating is reduced even without special protective measures.

According to the invention, the heating resistor of the electrical resistance heating element is rod-shaped, with the heating resistor being held by carrier plates that are located at the canal inlet side and the canal outlet side of the resistance heating element, and in which the flow canal continues. At its ends, the heating resistor positively engages the carrier plates.

In this context, rod-shaped heating resistor means a solid heating resistor whose length is distinctly greater than its width. Its cross-sectional shape can be selected randomly, and may also vary over its length. In the longitudinal direction, the heating resistor may also have two or more parallel sections that are connected to each other to form a current path.

The heating resistor includes an electrically conductive ceramic material for conducting the current. The conductive ceramic material may itself serve as the ceramic heating resistor, or may be provided as the jacket of a ceramic rod consisting of electrically non-conductive ceramic material. Such a heating resistor has high mechanical as well as electrical wear and thermal strengths, thereby making a long service life possible. In addition, the ceramic materials used in this device also have excellent properties regarding thermal and electrical conductivity. The flow of current through the heating resistor can be influenced by the conductivity of the ceramic material as well as by the geometry of the ceramic heating rod. By changing the conductive and non-conductive portions of its components, the conductivity of the ceramic material can be varied over a wide range. An additional advantage is due to the fact that, compared with known heating elements, higher temperatures and faster temperature changes are possible.

The electrical heating element according to the invention is intended, for example, for an air heater that can be operated with voltages of 48 V, as commonly used in ships and aircraft, or with 110 V, 230 V, or 380 V provided as line voltages by electrical power companies. For this purpose, the electrical resistance of the ceramic rod-shaped heating resistor must be sufficiently high. This can be achieved either by a high specific resistance of the electrically conductive ceramic material and/or by the geometric design of the ceramic heating resistor.

Preference is given to an embodiment of the electrical resistance heating element where the conductive ceramic material has a specific resistance of between 0.01 and 1.0 Ω-cm and/or where the ratio of the length to the cross-sectional area of the ceramic material of the heating resistor is between 1 and 500 cm⁻¹.

Because of the achievable specific resistance values and the operating voltages, oblong heating elements must be used for the resistance heating element according to the invention. In order to avoid costly electrical connection technology at the side of the heating resistor that is hotter during operation, i.e. at the canal outlet side of the electrical resistance heating element, a U-shaped rod-shaped heating resistor offers advantages. This doubles the length of the ceramic heating rod and therefore its electrical resistance value. In order to further increase the electrical resistance of the resistance heating element and to enlarge the contact area with the flowing gaseous medium, several preferably U-shaped heating resistors may be provided. Depending on the values of the available voltage supply and the desired power, they may be connected parallel or in series.

For establishing electrical contact, it serves the purpose if the ends of the rod-shaped heating resistors are coated with a layer of metal. This coating may be applied by means of vapor deposits or by sputtering, for example. Preferably, a metal paste is used that typically contains silver and possibly an additional precious metal, such as platinum or palladium.

In a preferred embodiment of the invention, the carrier plates holding the heating resistor are formed out of an electrically non-conductive ceramic material. Except for the electrical conductivity, the carrier plates have specific material characteristics that are similar to those of the ceramic heating rod.

In addition, the carrier plates preferably have recesses that match the cross-sectional shape of the heating resistor and serve to accept the ends of the heating resistor, and also have passages through which the flow of the gaseous medium are able to pass.

Preferably, the resistance heating element according to the invention has several ceramic heating rods that are arranged concentrically around the center of the heating element. With its front and rear end sections, each U-shaped heating resistor is held positively in the recesses of the carrier plates, with the carrier plates being fixed to each other by means of a common central pin extending in the axial direction. Through the recesses of the carrier plate on the canal inlet side, the heating resistors are connected electrically to each other and to the power supply lines. In addition, the flow of the medium is able to pass through the flow canal that extends between the two carrier plates and in which at least one heating resistor is located by passing through the passages of the two carrier plates.

In one embodiment of the resistance heating element according to the invention, the heating resistor is positively connected to the carrier plates. In order to hold the oblong heating resistors with the carrier plates in position, the carrier plates may be permanently connected to each other before, during, or after the sintering process. In contrast to ordinary electrical resistance heating elements, heating elements produced in this way are self-supporting and do not need to be guided or supported by an additional element. The carrier plates that hold the ceramic heating resistors on both sides must be able to withstand temperatures far in excess of 1000° C., for example, at the air outlet side of the resistance heating element.

The U-shaped ceramic heating resistor may have a flat or a structured surface. Preference is given to an embodiment of the invention where the heating resistor has indentations and/or raised areas in order to enlarge the surface area that is in contact with the gaseous medium, as compared with a flat surface. Besides providing as large a surface area as possible for the heat exchange with the flowing medium, this more complex geometry also has the effect of creating turbulence in the medium flowing past the heating resistor in the flow canal. This turbulence promotes the uniform and homogeneous heating of the entire flow of the medium.

Advantageously, the conductive ceramic material of the heating resistor consists of a mixed ceramic material with a conductive and a non-conductive component, with the conductive ceramic component having a positive thermal coefficient between 0 and 10,000 ppm/K. Preferably, molybdenum disilicide (MoSi₂) is used as the conductive ceramic component, and aluminum oxide (Al₂O₃) as the non-conductive ceramic component.

The preferably positive thermal coefficient should not rise too steeply because this might lead to the formation of hot spots. Also, a steep flank of the specific resistance should not occur within the operating temperature up to approximately 1500° C., as is the case with some electrically conductive ceramic materials with positive thermal coefficients. The mixed ceramic material may also contain additional additives for improving the sintering characteristics or the stability. It is also possible to partially substitute other insulating ceramic materials such as oxide or nitride or silicate ceramic material for the aluminum oxide. The specific resistance of the material can be set within a certain range by the proportion of the conductive component. A proportion of 20 to 30% of molybdenum disilicide in the mixed ceramic material proved to be ideal. This makes it possible to achieve specific resistances between 0.01 and 1.0 Ω-cm at room temperature that increase by a factor of three or more up to 1000° C.

The heating device according to the invention, for example for a hot air device, with a resistance heating element that is arranged in the flow of the gaseous medium and is surrounded by a heating tube, includes an electrical resistance heating element according to the invention. At one end of the heating tube, air or gas, for example, is blown in as a medium, and the heated flow of air or gas exits from the other end of the heating tube. The flow of air or gas may be generated by a blower provided on the heating device, or may be supplied to the heating device from an external pressurized reservoir.

Below, the invention is explained in detail with reference to two embodiments shown in the attached drawings. Additional characteristics of the invention are given in the following description of the embodiments of the invention in conjunction with the claims and the attached drawings. The individual characteristics of the invention may be realized either individually by themselves or in combinations of several in different embodiments of the invention. The embodiments shown are intended for the heating of a flow of air.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of a resistance heating element according to the invention;

FIG. 2 shows a top view of the carrier plates in FIG. 1;

FIG. 3 shows a perspective view of the heating resistor in FIG. 1;

FIG. 4 shows a second embodiment of a heating resistor with a smooth surface;

FIG. 5 shows a third embodiment of a heating resistor with a structured surface;

FIG. 6 shows a second embodiment of a carrier plate matching the heating resistors according to FIG. 5;

FIG. 7 shows a perspective view of a first heating device according to the invention; and

FIG. 8 shows a perspective view of a second heating device according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an embodiment of the resistance heating element 1 according to the invention with four heating resistors 2 in an axial section view, with only two heating resistors visible in the drawing. The heating resistors 2 are essentially rod-shaped and have front and rear end sections 3, 4. The rear end sections 4 are associated with an air inlet side 5, and the front end sections are associated with an air outlet side 6 of the resistance heating element 1. The heating resistors 2 are U-shaped and have a flat surface 7, ends 8, 8′, limbs 9, 9′, and a base 10 that connects the limbs 9, 9′. By means of an electrical bridge 11, the ends 8′ of the heating resistors 2 are connected in an electrically conductive way. Electrical supply lines 12 lead to the ends 8 of the heating resistors 2.

The heating resistors 2 are held by a front and a rear carrier plate 13, 14. The base 10 positively engages the front carrier plate 13 on the air outlet side, the two facing ends 8, 8′ positively engage the rear carrier plate 14 on the air inlet side. The carrier plates 13, 14 are fixed to each other by means of a central, preferably square pin 15. The heating resistors 2 extend in an air canal 16 that is surrounded on its face sides by the carrier plates 13, 14 and on its circumference by a heating tube (not shown) when the heating element 1 is installed in a hot air device.

As FIGS. 1, 2 show, the electrical resistance heating element has an oblong and essentially cylindrical shape. FIG. 2 shows a top view of the carrier plates 13, 14 consisting of circular flat plates that are made from an electrically non-conductive ceramic material. The Figure shows their inside surface associated with the heating resistors 2. The carrier plates 13, 14 have recesses 17 with holes 18 for accepting the front and rear end sections 3, 4 of the heating resistors 2. In addition, a central attachment hole 20 for the pin 15 connecting the carrier plates 13, 14 is provided. The holes 18 of the recesses 17 serve for the electrical connection of the heating resistors 2.

FIG. 3 shows a first embodiment of the U-shaped heating resistor 2 made of an electrically conductive ceramic material. The limbs 9, 9′ and the base 10 have a square cross-section and are located on one plane. The surface 7 is flat.

FIG. 4 shows a second variant of the heating resistor 2 shown in FIG. 3 where the limb 9 has a rectangular cross-section and the limb 9′ and the base 10 have a triangular cross-section, all together forming a cylinder segment that corresponds to one eighth of a straight circular cylinder. The surfaces 7 of the limbs 9, 9′ are flat and inclined towards each other.

FIG. 5 shows a third embodiment of the U-shaped heating resistor 2. The limbs 9, 9′ and the base 10 have an essentially triangular cross-section. They are designed so that together they form a cylinder segment that corresponds to one quarter of a circular cylinder. The surface 7 of the limbs 9, 9′ is structured. It has indentations 21 and raised areas 22 in order to enlarge the surface 7 for its contact with the flow of air. In addition, the structured surface 7 has the advantageous effect of creating turbulence of the flow of air, and thereby ensuring uniform heating thereof.

FIG. 6 shows the carrier plates 13, 14 that match the heating resistor 2 shown in FIG. 5. The carrier plates 13, 14 have a ring-shaped, essentially rectangular recess 17 and are suitable for a positive connection to the heating resistors 2. For this purpose, the heating resistors 2 are positioned in the corners 23 of the recess 17. In addition, the carrier plates 13, 14 have passages 19 for the flow of air, and the recess 17 has holes 18 for the electrical connection of the heating resistors 2.

The carrier plates 13, 14 matching the heating resistor 2 shown in FIG. 4 are not shown. Their design may be similar to that of the carrier plates 13, 14 shown in FIG. 6, with the shape of the recess 17 adapted to the different cross-section of the cylinder segment of the heating resistor 2.

FIGS. 7, 8 show two embodiments of hot air devices 24, 25 according to the invention. The hot air device 24 shown in FIG. 7 is a hot air device without an integrated blower that can be flange-mounted on a machine frame. The flow of air is supplied to the hot air device 24 externally. The hot air device 25 shown in FIG. 8 is equipped with an internal blower and may be used as a hand-held hot air device. Other than that, both devices are essentially similar in design. They have a housing 26 with a front heating tube 27. The resistance heating element 1 according to the invention (not shown in the Figure) is installed in the heating tube 27. The air outlet side 6 of the resistance heating element 1 points in the direction of the air outlet opening 28 of the heating tube 27, and the air inlet side 5 points towards the rear end of the housing 26. Via the housing 26, the heating resistors 2 of the resistance heating element 1 are supplied with electrical voltage.

Further features of the invention can be found in the description of preferred embodiments of the invention in connection with the claims and the drawings. The single features can be realized alone or several together in embodiments of the invention. 

1. An electrical resistance heating element for heating a gaseous medium, with at least one heating resistor that extends essentially in the longitudinal direction of the electrical resistance heating element, with the medium flowing past the heating resistor and being heated in the process, and with at least one flow canal that extends along the heating resistor and through which the medium is able to flow from a canal inlet side to a canal outlet side of the resistance heating element, with the heating resistor comprising an electrically conductive ceramic material for conducting the current, wherein the heating resistor is rod-shaped and is held by carrier plates that are located at the canal inlet side and the canal outlet side of the resistance heating element, with the flow canal continuing in said carrier plates, and with the heating resistor positively engaging the carrier plates at its ends.
 2. A resistance heating element as claimed in claim 1, wherein the conductive ceramic material has a specific resistance of between 0.01 and 1.0 Ω-cm.
 3. A resistance heating element as claimed in claim 1, wherein the ratio of the length to the cross-sectional area of the ceramic material of the heating resistor ranges between 1 and 500 cm⁻¹.
 4. A resistance heating element as claimed in claim 1, wherein the heating resistor is essentially U-shaped.
 5. A resistance heating element as claimed in claim 1, wherein the carrier plates are formed from an electrically non-conductive ceramic material.
 6. A resistance heating element as claimed in claim 1, wherein the carrier plates have recesses that match the cross-sectional shape of the heating resistor and serve to accept the ends of the heating resistor, and also have passages through which the flow of the gaseous medium is able to pass.
 7. A resistance heating element as claimed in claim 1, wherein the heating resistor is positively connected to the carrier plates.
 8. A resistance heating element as claimed in claim 1, wherein the heating resistor has indentations and/or raised areas in order to enlarge its surface for the contact with the flow of the gaseous medium, in comparison with a flat surface.
 9. A resistance heating element as claimed in claim 1, wherein the conductive ceramic material is a mixed ceramic material with a conductive and a non-conductive component, with the conductive ceramic component having a positive thermal coefficient, and with molybdenum disilicide (MoSi₂) used as the conductive ceramic component, and aluminum oxide (Al₂O₃) used as the non-conductive ceramic component.
 10. A heating device for heating a flowing gaseous medium, with an electrical heating element as claimed in claim 1 surrounded by a heating tube positioned in a flow of the gaseous medium, with the flow being generated by a blower provided on the heating device or being provided from an external pressurized reservoir. 